Abstract: The seat cushion (14) and the support (20) have respective ends articulated together by an assembly (26) forming a hinge with two essentially horizontal axes of articulation (Y1, Y2) so as to allow the seat cushion (14) to be pivoted between a normal position of use in which it is folded down on the support (20), and a tipped-up position in which it frees its normal space. The arrangement comprises members (44, 46) for retaining the seat cushion (14) in the normal position in the event of this seat cushion (14) experiencing an impact which would tend to lift its articulation end with respect to the articulation end of the support (20). As a preference, this retaining member comprise a hook (46) borne by the articulation end of the seat cushion (14) and cooperating with a crossmember (44) delimiting the articulation end of the support (20).